Fix and use RDF streaming

https://www.pivotaltracker.com/story/show/80671446
https://www.pivotaltracker.com/story/show/59236554
- Do we want to exclude the JCR namespace here? I'm guessing upstream machinery should already have dealt with it, but just to be really, really, really.
- Do you want to factor out a set of excluded namespaces?
- Shouldn't we be checking for namespace and not prefix? A ns can be registered under more than one prefix.